The present invention relates to a radio transceiver for providing duplex communication on a time division basis, and more particularly to a circuit for coupling a transmitter and receiver that operate in the same frequency band to a common antenna.
Certain radio communication systems provide for two-way or duplex communication by alternately transmitting and receiving in a common frequency band on a time-division basis. In this type of system, called a Time Division Duplex (TDD) system, the antenna needs to be connected at any given instant to either the transmitter or receiver, but not to both simultaneously.
One type of TDD circuit provides for connection of the transmitter and receiver to the common antenna by means of a so-called transmit-receive switch. The transmit-receive switch connects the antenna to either the transmitter or the receiver at any given time. The transmit-receive switch thus needs to be controlled by circuits that determine at any given time whether a particular time slot of a time-division multiple access (TDMA) signal is to be transmitted or received.
Several problems are presented by the introduction of a transmit-receive switch between the antenna, transmitter and receiver.
Transmit-receive switches, particularly those made of diodes or field effect transistors (FETs), can distort the modulation envelope of the transmitted signal.
A further problem introduced by the use of a transmit-receive switch is that transmit power may be reflected from the antenna back to the transmitter. This changes the effective impedance of the amplifier that is typically used as the final stage of the transmitter. This can degrade the amplifier""s intermodulation characteristics, therefore requiring the need to increase its power handling capability, which reduces efficiency.
A three-port device known as a circulator may also be used to implement a TDD transceiver. Similar to a duplexer, a circulator will pass the transmit signal to the antenna with low attenuation, but will not pass the transmit signal to the receiver.
One known limitation to using a circulator in a time division duplex radio is that any transmit power reflected by the antenna (i.e., not absorbed by the antenna) may be conducted back to the receiver. The receiver, in turn, must therefore have increased power handling capability for this eventuality. One solution to this problem is available when the transmitter and receiver operate in different frequency bands. In such a case, a receiver input filter may be used to reflect the transmitter power once again back toward the transmitter. However, such signals may be passed with lower attenuation by the circulator, with the again-reflected signal in turn appearing as an impedance mismatch.
In another solution to this problem, a switching device is connected between a load and the circulator or the receiver. The receiver is connected to the circulator during known times of receiver operation and the load is connected to the circulator during times of known transmitter operation. In this manner, the load absorbs power reflected by the transmitter.
However, this solution still requires the use of a switch that is synchronized to the specific state of the TDD radio. In other words, this solution still requires the use of a switch that potentially has the associated attendant problems with introducing distortion to the modulation envelope of the transmitted signal and/or power reflections.
Further difficulties exist when TDD systems are adapted for applications such as Local Multipoint Distribution Service (LMDS). These systems make use of modems to provide intermediate frequency (IF) signals to the respective transmitter and receiver. The transmit and receive modem signals are typically coupled to the TDD radio with a single coaxial cable. As such, LMDS transceivers typically utilize switches not only on the RF side for directing signals to and from the antenna port, but also on the IF side for directing signals to and from the bi-directional ports of the modem. At any given instant in time, the state to which the IF side switch is set is determined by whether the transceiver is to operate in the transmit mode or in the receive mode.
The present invention is a circuit for a Time Division Duplex (TDD) radio that eliminates the need for transmit-receive switches and the circuitry to operate them. The circuit includes a three-port circulator on the radio frequency (RF) side to couple the transceiver to an antenna. The circulator is connected to accept a radio frequency transmit signal from a transmitter section, with the transmitter section including an intermediate frequency (IF) input stage, an up-converter, and a power amplifier. A receiver port of the circulator connects to a receiver section, which includes a receiver low noise amplifier (LNA), a receive signal power detector, a down-converter mixer, and an intermediate frequency (IF) output port. Under normal conditions, the return loss looking into the antenna through the circulator is sufficient to prevent transmit power from saturating the receiver.
However, the receive signal power detector is coupled to a blanking circuit to control the operation of the receiver low noise amplifier. In particular, the detector and blanking circuit disable the receiver during conditions when the receiver may be susceptible to saturation, such as when excessive transmit power is being reflected back from the antenna. The detector and blanking circuit may blank the receiver by disconnecting an input amplifier from its supply voltage when input signal power levels exceed a predetermined threshold.
This arrangement therefore avoids a saturation condition in the receiver which might otherwise occur. For example, while this saturation condition does not necessarily damage the receiver, it places the amplifier in a state where an unacceptably long recovery time is needed before the receiver can again operate properly. This, in turn, means that the transceiver might not switch from the transmit mode to the receive mode in a required time frame. The detector and receiver blanking circuit therefore reduce susceptibility to this problem.
Optionally, on the modem or intermediate frequency (IF) side of the transceiver, a pair of isolators are used along with a second circulator or a hybrid coupler/splitter. This permits both the IF transmit and IF receive signals to be fed to the transceiver along a single coaxial cable to a common If port.
In other embodiments, the transmit and receive signals may be carried on separate cables to the respective transmit and receive sections of the transceiver.